The present invention relates to a shaped beam array antenna, and particularly to that to be used in a microwave to millimeter-wave band for generating a cosecant square beam.
In a conventional shaped beam array antenna consisting of traveling-wave type array antennas, the cosecant square beam is shaped by optimizing coupling factors and locations of all antenna elements of the traveling-wave type array antenna so that a desired excitation amplitude distribution and a desired excitation phase distribution be obtained.
FIG. 8A is a perspective view illustrating an example of the conventional shaped beam array antenna and FIG. 8B is a partial magnification of FIG. 8A. In the example of FIG. 8A, the cosecant square beam is realized making use of wave-guide slot array antennas as the traveling-wave type array antennas, whereof the excitation amplitude distribution, the excitation phase distribution and the array radiation pattern are illustrated in FIGS. 9A, 9B and 9C, respectively.
Referring to FIG. 8A, the conventional shaped beam array antenna consists of a wave guide 2 and a terminal dummy 3 provided at an end of the wave guide 2. A wall of the wave guide 2 having a rectangular section is provided with a plurality (N) of slots 1.sub.1 to 1.sub.N each functioning as an antenna element. In FIG. 8A, a fringe 202 provided at the other end of the wave guide 2 is further depicted together with a center line 201 of the slotted wall of the wave guide 2.
Each of the slots, an n-th slot 1.sub.n (n=1 to N), for example, is configured parallel to the center line 201 with each offset distance X.sub.n as shown in FIG. 8B. By controlling each offset distance X.sub.n, the coupling factor of each slot 1.sub.n is adjusted in order to realize the desired excitation amplitude distribution such as illustrated in FIG. 9A, for example.
In the example of FIGS. 9A and 9B, the wave guide 2 has twenty slots and the element numbers 14 to 33 correspond to the slots 1.sub.1 to 1.sub.N (N=20) of FIG. 8A. The element number 14 represents the slot 1.sub.1 nearest to the fringe 202, that is, to the feeder side, while the element number 33 represents the slot 1.sub.N farthest from the feeder side.
Returning to FIG. 8B, the resonance length of the slot depends on its offset distance from the center line 201. Therefore, slot length L.sub.n of each slot 1.sub.n is prepared to be the same with the resonance length determined by each corresponding offset distance X.sub.n.
Furthermore, by controlling each separation d.sub.n (n=1 to N-1) of FIG. 8B between two successive slots 1.sub.n and 1.sub.n+1, the desired excitation phase distribution is realized such as illustrated in FIG. 9B.
By thus realizing the excitation amplitude distribution and the excitation phase distribution of FIGS. 9A and 9B, the array radiation pattern of FIG. 9C is obtained, wherein the radiation angle 90.degree. represents an upper vertical direction towards the terminal dummy 3 of FIG. 8A and the radiation angle -90.degree. represents a lower vertical direction towards the feeder side.
In the array radiation pattern of FIG. 9C, the cosecant square beam is obtained in an effective radiation angle range of -30.degree. to 0.degree..
However, there are following problems in the conventional shaped beam array antenna as above described.
First, there are needed antenna elements capable of adjusting their coupling coefficients in a wide range for realizing the cosecant square beam. The reason is that the coupling coefficients should be high in the middle and become lower towards both ends of the antenna array in order to obtain the excitation amplitude distribution such as illustrated in FIG. 9A for generating the cosecant square beam.
Second, high precision is needed for fabricating the shaped beam array antenna. The reason is that antenna elements each having its own size a little different with each other should be ranged with separations each determined a little differently with each other in order to obtain the necessary excitation amplitude distribution and the necessary exitation phase distribution.
Third, the conventional shaped beam array antenna cannot be trimmed after once designed or fabricated. The reason is that the cosecant square beam is realized by controlling the phase and amplitude of everyone of the antenna elements, and so, effect to the array radiation pattern of the phase and amplitude of an individual antenna element cannot be specified independently.